FOLIA HISTOCHEMICA REVIEW ET CYTOBIOLOGICA Vol. 53, No. 2, 2015 pp. 105–119 The Hippo pathway in colorectal cancer Piotr M. Wierzbicki, Agnieszka Rybarczyk Department of Histology, Faculty of Medicine, Medical University of Gdansk, Gdansk, Poland Abstract Colorectal cancer (CRC) is one the most frequently diagnosed neoplastic diseases worldwide. Currently, aside from traditional chemotherapy, advanced CRCs are treated with modern drugs targeting cellular components such as epithelial growth factor receptor (EGFR). Since up to 70% of metastasized CRCs are drug resistant, the description of recent progress in cellular homeostasis regulation may shed new light on the development of new molecular targets in cancer treatment. The Hippo pathway has recently become subject of intense investigations since it plays a crucial role in cell proliferation, differentiation, apoptosis and tumourigenesis. Components of the Hippo pathway are deregulated in various human malignancies, and expression levels of its major signal transducers were proposed as prognostic factors in colorectal cancer. In this review we focused on recent data regarding Hippo pathway, its up-stream signals and down-stream effectors. Hippo negatively regulates its ma- jor effectors, YAP1 and TAZ kinases, which act as transcriptional co-activators inducing expression of genes involved not only in tissue repair and proliferation but are also oncoproteins involved in tumour development and progression. The deregulation of Hippo pathway components was found in many malignancies. The interactions between Hippo and Wnt/b-catenin signalling, crucial in the maintenance of cell homeostasis, have been described in relation to the control of intestinal stem cell proliferation and CRC development. The recently discovered positive feedback loop between activated YAP1 and increased EGFR/KRAS signalling found in oesophageal, ovarian and hepatocellular cancer has been related to the CRC progression and resistance to EGFR inhibitors during CRC therapy. (Folia Histochemica et Cytobiologica 2015, Vol. 53, No. 2, 105–119) Key words: CRC; Hippo pathway; YAP1/TAZ; Wnt signalling; EGFR/KRAS; intestinal stem cells Abbreviations: AJ — adherent junction, AMOT — tumor suppressor kinase 1/2, Lgr5 — leucine-rich angiomotin, APC — adenomatous polyposis coli, repeat-containing GPCR5, LPR — Lipoprotein AREG — amphiregulin, Axl — Axl receptor tyrosine receptor-related protein, MOB1 — MOB kinase kinase, BMI1 — BMI1 polycomb ring finger proto-on- activator 1, MST1/2 — serine/threonine kinase 4/3, cogene, β-TrCP — beta-transducin repeat containing NF2 — neurofibromin 2, Oct-4 — octamer-binding E3 ubiquitin protein ligase, CK1 — casein kinase 1, transcription factor 4, RASSF — Ras association CRC — colorectal cancer, CTGF — connective tissue (RalGDS/AF-6) family member, SAV1 — Salvador growth factor, DSS — dextran sodium sulphate, DVL family WW domain containing protein 1, STRIPAK — Dishevelled protein, ECM — extracellular matrix, — striatin interacting phosphate and kinase complex, EGFR — epithelial growth factor receptor, EMT — TAZ — tafazzin, TCF4 — transcription factor 4, epithelial-to-mesenchymal transition, FAT — FAT TEAD — TEA domain family member, TJ — tight atypical cadherin, FZD — Frizzled protein, GPCR junction, b-TrCP — b-transducin repeat-containing — G protein coupled receptor, GSK3 — glycogen protein, VEGF — vascular endothelial growth factor, synthase 3, ISC — intestinal stem cell, KO — knock WB — Western blot technique, YAP1 — Yes-asso- out, KRAS — Kirsten rat sarcoma, LATS1/2 — large ciated protein 1 Colorectal cancer Correspondence address: P.M. Wierzbicki, Ph.D. Department of Histology, Faculty of Medicine Medical University of Gdansk Colorectal cancer (CRC) is the third most commonly Debinki St. 1, 80–211 Gdansk diagnosed cancer in males and the second in females, tel.:+48 58 349 15 01, +48 58 349 14 37, fax: +48 58 349 14 19 with an estimated 1.4 million cases and 693,900 deaths e-mail: [email protected] occurring in 2012 [1]. Five-year relative survival ranges ©Polish Society for Histochemistry and Cytochemistry Folia Histochem Cytobiol. 2015 www.fhc.viamedica.pl 10.5603/FHC.a2015.0015 106 Piotr M. Wierzbicki, Agnieszka Rybarczyk Figure 1. The mammalian Hippo pathway. Drosophila orthologue genes are shown below the names of the mammalian proteins. When the Hippo pathway is not active (e.g. during cell injury and repair or in cancer), the active effector pro- teins, YAP1 and TAZ, interact with TEAD1-4 transcription factors and promote transcription of genes involved in cell proliferation. During normal cell homeostasis, when the Hippo pathway is active, YAP1 or TAZ are inhibited due to their phosphorylation by core components of the Hippo pathway (SAV1, MST 1/2, LATS1/2 — shown in the central rectan- gle). In a phosphorylated form cytoplasmic YAP1/TAZ may interact with (i) the 14-3-3 protein, (ii) components of cell junctional complexes like AMOT or b-catenin, or (iii) may be degraded in proteasomes. YAP1/TAZ can be also regulated by other mechanisms such as cell polarity, GPCR signalling or ECM stiffness as described in the body text. Acronyms are explained on the first page from more than 90% in patients with stage I disease tics and so called biological drugs [6]. In this review to about 10% in patients with stage IV disease [2]. we also aimed to bring closer the possible associations Treatment of CRC is one of the most expensive when between the Hippo pathway and drug resistance of diagnosed at later stages when prognosis is generally CRC cells. poor [3]. Although the knowledge of the background and the development of CRC has recently increased, The Hippo pathway there is a high need for more studies to found out well working prognostic, survival and diagnostic markers. The Hippo pathway is an important regulator of cell Serum markers for routine CRC diagnostic such as CEA proliferation, growth and apoptosis [7, 8]. Moreover, (carcinoembryonic antigen) and CA 19-9 showed good it controls tissue homeostasis, organ size and stem cell prognostic values and have been used as CRC tumour functions. Its deregulation is frequently observed in predictors [4]. The aim of this paper was to provide many human cancers, suggesting that alterations of rationale to consider the expression of Hippo pathway Hippo signalling are connected with tumour initia- genes as possible new prognostic genes in CRC. tion and/or progression [9–11]. Hyperactivation of The modern chemotherapy has adapted molecu- the Hippo pathway downstream effectors — YAP1 lar findings in many cancers to focus on activation/ (Yes-associated protein 1) and TAZ (transcriptional /inactivation of cancer-related intracellular pathways co-activator with PDZ binding motif) may contribute to eliminate or decrease expansion of tumour cells. to the development of cancer, however, their activa- Such drugs which target vascular endothelial growth tion may also play a positive role in stimulating tissue factor-A (VEGF, Bevacizumab) or epidermal growth repair and regeneration following injury [12, 13]. factor receptor (EGFR, Cetuximab and Panitumumab) The general scheme of the Hippo pathway and inter- have been introduced for the treatment of CRC [5]. actions of mammalian Hippo components with up- However, it was noted that 50–70% of advanced CRC stream signals and effectors are presented in Figure 1 cases were resistant to both classical chemotherapeu- and listed in Table 1. ©Polish Society for Histochemistry and Cytochemistry Folia Histochem Cytobiol. 2015 www.fhc.viamedica.pl 10.5603/FHC.a2015.0015 Hippo pathway in colorectal cancer 107 Table 1. Names and main functions of the most important Hippo pathway genes and proteins Mammalian gene Drosophila gene, protein Protein name, aliases Protein function NF2 Mer, Merlin Neurofibromin 2, Merlin, ACN, Signal transduction between cell-membrane SCH, BANF and cytoskeletal proteins WWC1 Kibra, ortholog WW and C2 domain containing 1, Phosphoprotein involved in cell polarity, KIBRA, HBEBP3, HBEBP36 mitosis and cell migration [105] FRMD6 Ex, Expanded FERM domain containing 6, EX1, Protein linking cytoskeleton with plasma Willin membrane [106] STK4 Hpo, Hippo Serine/threonine kinase 4, Cytoplasmic kinase involved in stress- MST1, KRS2 -induced mitogen-activated protein kinase cascade STK3 Serine/threonine kinase 3, Kinase activated by proapoptotic molecules MST2, KRS1 for the promotion of chromatin condensation during apoptosis, cardiomyocyte prolifera- tion [107], regulation of osteoblast/osteoclast differentiation [108] RASSF1 Ras association (RalGDS/AF-6) Signal transducer, microtubule stabilization, domain family member 1, cell cycle arrest [109] RASSF1A, NORE2A SAV1 Sav, Salvador Salvador family WW domain Regulation of protein degradation, RNA containing protein 1, SAV, WW45 splicing and DNA transcription LATS1 Wts, Warts Large tumour suppressor kinase 1, Serine/threonine kinase involved in mitosis: WARTS interacts with CDC2/cyclin A LATS2 Large tumour suppressor kinase 2, Kinase which interacts with centrosomal pro- KPM teins aurora-a and ajuba during mitosis [110] MOB1A Mats MOB kinase activator 1A, MOB1, Protein involved in mitotic exit network [111], MATS, microtubule stability control [112] YAP1 Yki Yes-associated protein 1, YAP, YKI Downstream effector of Hippo pathway involved in development, repair and homeo- stasis TAZ Tafazzin, EFE, Taz1 Non-specific phospholipid-lysophospholipid transacylase involved in cardiolipin turnover [113] Discovery and function of the Hippo pathway by